NGC 4548 bar characterization based on P4 isophotal ellipticity profiles. Top panel: image of NGC 4548, in magnitude scale with range 17−25 μ_{3.6 μm}(AB). The frame is aligned in a way that the y-axis points north and east is to the left. Axes are in units of arcsec. The ellipse corresponds to the maximum ellipticity isophote, while the straight line shows the visual estimate of the bar length and PA. Middle panel: ellipticity profile, with the maximum highlighted with a vertical line. Bottom panel: PA profile, with r_ε displayed with a vertical line.